Seasonality of Life Histories in Tropical Birds : Circannual Rhythms and Zeitgeber

Abstract

In his seminal paper about potential circannual rhythms, Aschoff (Stud Gen 8:742–775, 1955) predicted their presence in species that live in relatively constant, tropical environments. Detailed follow-up investigations largely supported this idea, but life-history stages (such as breeding, molt, migration) of wild tropical species can be rhythmic or arrhythmic. We present some classical findings on circannual biology of tropical species and provide ecological and evolutionary context by reviewing the diverse patterns of seasonality in tropical environments. We relate this to the seasonality of life histories in tropical birds and discuss the potential Zeitgeber of circannual rhythms, which are still elusive. We emphasize the diversity of tropical environments, roughly covered by the biome concept, which range from extremely erratic and unpredictable to very predictable environments. Predictable environments, which arguably favor evolution of circannual rhythms, are characterized either by a rather constant climate throughout the year or by a pronounced and regular climatic seasonality. Colwells’ (Ecology 55:1148–1153, 1974) concept of predictability, constancy, and contingency allows a quantification of such diversity of tropical biomes and forms the basis of our discussion of the seasonality of life histories in tropical birds.